1. Field of the Invention
The subject invention relates to a method for detecting a loss of refrigerant in a vehicle air conditioning system.
2. Description of the Prior Art
Air conditioning systems for motor vehicles typically include a refrigerant circuit having a compressor, a condenser, an evaporator, an accumulator, and a compressor. The compressor includes a motor for circulating the refrigerant through the circuit. The refrigerant often includes a lubricating oil for lubricating the compressor during operation. The compressor can overheat and bum out if the refrigerant and the lubricating oil become depleted.
Heavy duty trucks require durable air conditioning systems for cooling the operator cabin. These air conditioning systems are required to operate consistently over hundreds of thousands of miles. To meet this requirement, a loss of refrigerant from the circuit must be identified before the compressor can be damaged from low refrigerant. Therefore, a need exist for accurately detecting a change in refrigerant level within the circuit before the compressor can be damaged from low levels of refrigerant.
One such device for detecting low refrigerant levels is disclosed in U.S. Pat. No. 5,457,965 to Blaire et al. The Blair et al patent discloses an apparatus for detecting a partial loss of a refrigerant fluid by measuring the temperature difference of the refrigerant fluid between an evaporator inlet and an evaporator outlet. A controller measures the temperature difference against a predetermined value and interrupts power to compressor at a low refrigerant charge. Alternatively, the Blair patent discloses an apparatus for detecting a low refrigerant level by calculating saturation temperature of the refrigerant by measuring temperature and pressure in the gas phase between the compressor and the condenser. The calculated saturation temperature is compared with a predetermined saturation temperature for determining if a superheat condition exists. A third alternative disclosed in the Blair et al patent includes a pressure switch, which is located between the accumulator and the compressor. A clutch is cycled at preset low and high pressures of refrigerant as measured in the gas phase between the accumulator and the compressor.
Although the disclosed concepts provide protection for the compressor, several shortcomings exist. First, the superheat calculation requires significant computer resources due to the complexity of the calculation. Also, simply determining the temperature difference between the inlet and the outlet of the evaporator, without more, can give inaccurate superheat readings reducing the effectiveness of the system. Further, no provisions are made for determining partial loss of refrigerant, which would ultimately lead to required maintenance, but would allow continued operation of the air conditioning system. Still further, measuring the refrigerant pressure in the gas phase has not proven to be an accurate indicator of low refrigerant charge at broad ranges of ambient temperatures.
A need exists for a refrigerant loss detection method that would accurately detect a complete loss of refrigerant as well as a partial loss of refrigerant. A method for refrigerant loss detection that would both provide a visual signal of refrigerant charge and terminate power to the compressor upon critically low refrigerant charge would be preferable to the prior art.